Made for Walking Article

Image: Pederpes finneyae

A fossil found in 1971 has been newly identified as the earliest known animal built to walk on land, a salamander-like creature that marked a previously unknown stage in the evolution of fish into the ancestors of all vertebrates alive today.  The toothy animal, Pederpes finneyae, lived between 348 million and 344 million years ago in what is now Scotland.  It was perhaps a meter long (about three feet), and probably split its time between the water and land where it walked on four feet.  It trudged through the swamp catching anything that moved.

The creature’s nearly complete fossil skeleton had lain, mislabeled as a fish, in a Scottish museum since its discovery 31 years ago. Further work on the fossil in the 1990s revealed it had legs.  The identification helps close a hole in the early fossil record of a group of creatures called tetrapods.  The gap, or Romer’s Gap — named for the late Harvard paleontologist Alfred Sherwood Romer — had stumped scientists seeking to chart the evolution of the first four-limbed creatures with backbones.  Tetrapods were the first animals known to walk the Earth and are the ancestors of today’s mammals, reptiles, amphibians and birds.  The discovery of a nearly complete skeleton in the middle of Romer's Gap would help in establishing the pattern of evolutionary change among early tetrapods.

The earliest tetrapods appeared roughly 365 million years ago when scientists believed they used their paddle-like feet to scoot around underwater.  Only later did they emerge to use those rudimentary feet to walk on land.  Scientists knew of no other fossils, until Pederpes, that represented any sort of intermediate stage between the aquatic and terrestrial tetrapods. The fossil record picks up again 20 million to 30 million years later with a variety of more modern-looking animals with feet and legs that were clearly for walking on land.

By the time Pederpes appeared, its feet did not stick out straight from the body, as was the case in earlier tetrapods that used their limbs as paddles.  Instead, they pointed forward, suggesting they were built for use on land. Later tetrapods elaborate on that form of foot construction.  Finally, with the discovery of animals such as this, paleontologists are beginning to get some actual data as opposed to speculation.

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